Vitrifiable enamel



Patented Deit 2,225,162 VITRIFIABLE ENAMEL Alden J. nevi-up, Elizabeth, N. 1., asslgnor to a. I. I

du Pont de Nemours & Company, Wilmington, De l., a corporation of Delaware No Drawing. Application February 2s,

rial No. 192,965. Renewed September 13,

This invention relates to the decoration of surfaces of ceramic articles by use of low meltin vitriflable'enamels. More particularly th invention relates to the application of low melting vitriflable enamels, usually pigmented or opaciiied, to the surfaces of glass objects for decorative purposes.

My co-pending applications S. N. 131,528 and S. N. 132,660, filed March 17, 1937 and March 23, 1937, respectively, describe general methods for preparing and usingvitrifiable enamels for application to glass, which enamels are characterized by a high resistance to chemical attack. My present invention is, in part, an improvement on and an addition to the discoveries of my above co-pending applications and is concerned principally with vitriflable enamels having a remarkable resistance to attackby sulfur-containin materials.

' Vitriflable enamels known heretofore which are fusible at temperatures low enough to permit their practical use on glass are notably lacking in resistance to chemical attack. Previous improvements in the art of making vitriflable enamels, as described in my above co-pending applications and also in my co-pendin'g applications S. N. 65,935; 131,529 and 141,188, filed February 26, 1936; March 1'7, 1937; and May 6, 1937, respectively, have resulted in greatly improving the stability of such enamels toward attack by acids, alkalies and cleaning agents.

However, the problem of making low melting.

enamels, which are resistant also to attack by sulfur-containing materials has thus far resisted every effort at solution.

The lack of resistance to staining-by sulfurcontaining materials has constituted a serious hindrance to the full and effective commercial utilization of vitriflable enamels on glass. As an outstanding example of this, in the field of glass tableware, attempts have been made heretofore to use vitrifiable enamels to produce brilliant and varicolored decorations on such ware, which attempts failed because of the fact that the enamels used were stained brown or gray by contabt with food materials containing sulfur and thus lost their original attractive appeararice, becoming undesirable in the eyes of the users. Numerous efforts have been made to discover low melting vitrifiable enamels suitably resistant to the attack of sulfur-containing food materials. However, to my knowledge, all of these efforts have resulted in failure and no one has succeeded'prior to the present invention in solving this problem in a satisfactory manner. i

rule.

It is known in the arts of ceramics and vitrifiable enamels .that in general the resistance of such an enamel to any form of chemical attack may be increased by-alteringthe composition so as to increase its melting point. It is also generally known that, conversely, any alteration in the composition which tends to lower the fusion point of the enamel will also tend to reduce its chemical resistance. This is a general rule, but

I find that it is not universally valid. My discoveries constitute specific exceptions to this In view of this generally accep'ted relationship between the fusion temperature of a vitrifiable' enamel and its chemical resistance, it has generally been the practice to'employ relatively high melting enamelswhen a highdegree of resistance is desired. In the decoration of glassware, however, the extent to whichthe resistance of the vitrifiable enamel may be increased by this method is limited in' that if the melting point of the enamel is increased beyond the deformation temperature of the glass, which is generally around 600 C., the enamel will not mature properly on the glass and is therefore of little' utility. Ordinarily the chemical resistance of enamels known heretofore which have'a fusion temperature approximately that of the deformation point for glass is not adequate and, in the case of resistance toward attack by sulfurbearing compounds, such enamels are of little practical value. 0n the other hand, the more fusible enamels, e. g., the usual lead silicate and lead borosilicate enamels, which can bereadily applied to glassware are of little or no practical value since they'possess little resistance to chemical attack. By the exercise of my present invention, however, it is possible to prepare vitri-Y fiable enamels which will mature to a glossy smooth surface at temperatures below the rangeat which glass objects deform, and yet possess excellent resistanceto attack by food or other materials or gases containing sulfur or its compounds.

Accordingly, it is an object of my invention to prepare low melting lead silicate and lead borosilicate vitrifiable enamels which are highly resistant especially to attack by sulfur-bearing materials. A further object is to produce low melting vitrifiable enamels consisting principally of silica, lead oxide, titanium dioxide and lithium oxide, which may or may not contain boric oxide, which enamels mature to an excellent glossy vsur-- face at practical glass decorating temperatures I and, in addition to having a high resistance to acids and alkalies, are especially resistant to attack by sulfur-containing substances. A still further object is the provision of improved low melting vitriflable enamels which may be applied for decorative purposes on glass intended for use 5 in contact with foods and are highly resistant to staining and attack by sulfur-bearing food materials, e. g., eggs. These and other objects will be apparent from the ensuing description of my invention.

I have discovered that if the oxides of lithium and titanium are used together as ingredients of a. lead silicate or lead borosilicate flux, enamels may be obtained from such a flux which mature to an excellent glossy surface at practical glass decorating temperatures, which enamels have good resistance to acids and alkalies and at the same time are virtually proof against staining by contact with sulfur-bearing materials. This surprising result has not been observed in any other enamel composition for glass and appears to be unique for those compositions in which the oxides of lithium and titanium are both present.

The' alkali metal oxides are generally regarded in the ceramic and glass arts as being substantially the equivalent of each other since the general experience heretofore in the use of these oxides has'been that nosharp change in the properties is brought about by the substituever, I have now discovered that the alkali met 1 oxides, when used in low melting vitrifiable enamel compositions containing titanium, are not the equivalent of each other in their effect upon sulfide resistance. Thus, while the use of lithium oxide in combination with titanium oxide produces an enamel which is virtually proof against staining by sulfide-bearing materials, other alkali metal oxides, e. g., sodium oxide and potassium orn'de, do not exert this effect when used in combination with titanium oxide. This discovery is indeed surprising and is clearly unexpected in view of the generally accepted belief that all alkali metal oxides are approximately equal in their efiect in compositions of the present type.

In practicing my invention it is essential that the oxides of lithium and titanium be used together as ingredients of the flux in order that the 0 enamels resulting therefrom may possess the sulfide resistance characteristic of my improved vitrifiable enamels. Other alkali metal oxides may be used, however, in addition to theoxide of lithium, but the eflect of such other oxide is no more than that which would be generally expectedin view of the previously known effect of such oxides in such compositions. It is not known why the joint use of lithium and titanium oxides'should have the effect which I have discovered but I have found that this effect is obtained only through their simultaneous presence in the composition and not through the use of either substance separately. 'Both lithium and titanium oxides have been previously used as ingredients of enamel compositions. Generally, the titanium oxide, which is commonly regarded in the art as being a refractory oxide, has been used as a milled-iningredient and is not meltedinto the flux. So far as I am aware, no one has ever before-used the combination of these oxides as melted-in ingredients of the flux from which the present improved enamels maybe obtained. Furthermore, I have found no reference in published literature which would suggest the use of. these oxides either singly tion of one alkali metal oxide for another. Howor in combination as a means of imparting sultide-resistance to a vitrlflable enamel composi tion. Sulfide resistance is a highly specific property and is probably related to a large extent to the amount of lead and other heavy metals in the composition and to the chemical combinations of these metals in the finished enamel. My vitrifiable compositio which are also excellently resistant to acids and alkalies. are not readily attacked or stained by sulfur-bearing materials even under severe exposure conditions. It is therefore evident that my inventon constitutes a notable improvement over previous advances in this field, such as those described in .my above mentioned co-pending applications.

As examples of batch compositions for low melting lead silicate and lead borosilicate fluxes of superior resistance to sulfur-bearing materials, containing titanium oxide and lithium oxide, melted in as ingredients of the flux, the following typical batches are given:

' Sodium silicate: 23.5'% NazO, 76.5% 310:

After melting, the fluxes or molten glassy materials resulting from the foregoing batch compositions will have the ,following calculated compositions:

Calculated Compositions Batch A Per cent Red lead I m 55.1 Flint V 35.1 Titanium dioxide I 2.8 Lithium carbonate 7.0

Batch B Per cent Red lead 52.3 Flint 36.9 Titanium dioxide 4.3 Lithium carbonate 3.2 Soda ash 3.3

Batch C Per cent Red lead 31.4 Flint 16.0 Titanium dioxide 4.1 Lithium carbonate 5.9' Cadmium carbonate 3.0 Sodium. silicate 33.1 Boric acid 6.5

Batch D Per cent Red lead 50.5. Flint 19.0 Titanium dioxide v 2.4 Lithium carbonate 4.3 Sodium silicate 19.9 Alumina hydrate 3.9

' table:

The ranges in which the calculated contents of the principal ingredients ofthe improved fluxes of my invention will lie are given in the following Ranges for Principali'luz Components Per cent PbO -60 SiOz 30-50 Tim 1-6 L120 1-4 All of these .percentages are by weight. and are based upon the total weight of the melted flux.

The above table of principal flux components will be of assistance if it is desired to depart from the specific examples given above which are to be understood as being illustrative and. not restrictive. It willbe understood that in the application of this table, optimum resistance to sulfur-bearing materials will be obtained by ad,- iusting the proportions of refractory oxides, e. 'g., silica, or by adding other refractory oxides, such as alumina, in amounts such that the melting I pointof the resulting flux is no lower than necessary to secure a glossy surface when the glassenamelmade therefrom is applied to glass-ob jects and fired under commercial decorating conditions. This statement is based uponmy ex- .perience with these novel fluxes as well as upon the generally recognized rule stated above regarding the inverse relationship between the fusion temperature and chemical resistance.

-In departing from the specific examples given above, I have discovered that an increase of the. total alkalies is beneficial to sulfide-resistance as the lead oxide content is diminished simultaneously so as to maintain the fusion temperature unchanged. However, if too much alkali is'presentthe enamel may crack or craze on the glass. It isgenerally beneficial from an economic stand point and frequently not detrimental to the glass enamel-properties to include other alkali metal oxides, e. g., sodium oxide, in addition to lithium oxide in my flux compositions containing titanium oxide. Of the alkali metal oxides, I prefer to employ amixture of equimolecular weights of lithium and sodium oxide over any other mixture of lithiumand sodium and/or potassium oxides. An excess of titanium oxideshould be avoided because, when an execs is employed, it may separate out at high'application temperatures, causing a variable yellow opacity and devitrification. I usually prefer to use'little and sometimes no boric oxide in my improved flux compositions because its presence is detrimental to the acid resistance produced by titanium oxide as described in my co-pending application, S. N.

132,660, filed March 23, 1937. u 7

The present fluxes may be prepared by the usual processing methods employed with other glass-enamel fluxes since the merit of these fluxes depends essentially upon the chemical composition rather than on particular. details of processing. The materials comprising the batch may be mixed together and melted by the application I of heat in a refractory crucible. After frothing vceases and proofs taken from the melt with an iron rod are clear and transparent, indicating that the titanium oxide and the refractory ma-. -terials have dissolved in the melt, the flux is poured into water and thereby. fritted. The coarsely fritted flux obtained may then be ground together with ceramic pigments in the usual proportions and manner to make vitrifiable enamels which are then ready to be applied to glass surfaces. In the mode of handling, application and the temperature and method of melting, my new compositions containing both titanium and lithium oxide are similar to the ordinary lead boro-- silicate compositions. My new products diifer sharply from such old compositions only in their greater superiority to resistance to attack by sulfur-bearingmaterials and to other chemical attack which property renders them useful in broad new fields.

' In order todemonstrate the superior resistance of my improved low melting enamels, various articles werecoated with enamels prepared from the batch compositions designated above as A,

r B andC. These vitrifiable enamels, after application and firing on the objects to be decorated,

'were subjected to tests to determinetheir resistance to chemical attack, the natureofthese tests being specified in the following examples:

' Example I -A yellow enamel composition was prepared from the flux of Batch A 'as given above by grinding therewith 4%. by weight of yellow lead antimonate ceramic pigment. Glass saucers were coated with this yellow'enamel and fried eggs were placed and allowed to remain thereon for one hour. For comparison purposes-glass saucers were coated with similar yellow enamels 'prepared from the. ordinary lead borosilicate fluxes and with other yellow enamels prepared from the fluxes described in myco-pending applications S. N. 65,935; 131,528; 132,660 and 141,188 mentioned above, and these decorated saucers were then subjected to the same'test. At the end of one hour the ordinary lead borosilicate enamels were blackened in large blotches and the enamels prepared according to my co-pending applica- These.

tions developed gray-brown blotches. stains could not be removed entirely with soap and water. ,The improved enan'iels containing titanium dioxide, and lithium oxide, 1 however, were practically unafi'ected and were found to be en- .tirely free from any stain or blemish after the.

oxide and sodium oxide; those of S. N. 131,528-

contained lead oxide, silica, boric oxide and melted-in titanium dioxide; and the enamelsoi' S. N.

132,660 contained lead oxide, silica, melted-in u tanium dioxide and sodiumoxide. These enamground in with the flux are cobalt aluminate blue, 1 lead antimonate yellow, and the like. It may be els possess a high resistance, especially to attack by acids and alkalies. The enamels 'of S. N. 141,188 were of the lead borosilicate type and contained melted-in zirconium dioxide which imparted to the flux exceptional resistance to attack by alkalies.

Hot fried eggs have been found to be the most severe reagents for determining the resistance of an enamel to sulfur-bearing materials. In some cases enamels of the older types may withstand contact with eggs in other forms but invariably they are blemished or blackened by contact with hot fried eggs. In view of the severity of the test described in Example I, it is apparent that the results obtained demonstrate the remarkable superiority of my new enamels as regards resistance to sulfur-bearing materials.

Example II 'Pieces of glass coated with the respective enamels in Example I were moistened and exposed to an atmosphere of hydrogen sulfide gas. The enamels made with the usual lead borosilicate fluxes became black or dark brown immediately upon contact with the hydrogen sulfide atmosphere. Those prepared according to my copending applications S. N. 65,935; 131,528; 132,660 and 141,188 remained unchanged at first but showed a noticeable darkening at the end of one hour. In contrast, the present improved enamels containing both titanium and lithium oxides showed no darkening over periods of exposure as long as 16 hours.

A wide variety of chemical compounds may be selected to secur the elements indicated in the calculated flux compositions set forth above. The necessary lead oxide may be obtained'either from white lead, litharge, red lead, lead nitrate or similar compounds. If desired, two or more lead compounds in any desired proportion may be ut lized as a source of the lead oxide. The sili- Y need be maintained in carrying out the melting of the components to produce my improved fluxes. It is only necessary to bring the batch mixture to its melting point and hold it there for a sufllcient period of tim sothat the components react and form a molten glass which is reasonably free of unreacted or undissolv'ed materials. The molten flux is usually poured into water in the fritting operation. It may also be poured out upon any convenient clean surface if desired. After pouring, the flux is crushed and ground or otherwise comminuted asdesired' in accordance with any suitable procedure.

Any of the known ceramic pigments may be introduced into my improved fluxes by milling or inding. opacifiers are lead molybdate, zirconium oxide or titanium oxide. Other pigments which may be preferred in some cases to pigment the flux by Examples of suitable pigments or" melting in a coloring compound directly with the batch. Such compounds may be cobaltoxide, lead chromate, or the like. The exact proportion of pigment to be incorporated depends chiefly upon the color desired and the nature of the pigment. Generally the proportion will lie within the range of 2 to of pigment based upon the total weight of the flux. I

In the appendedclaims, the term lead silicate," or vitrifiable enamels of the lead silicate typ IS used to define my improved enamels which contain lead oxide and silica together with melted-in titanium dioxide and lithium oxide. The term lead borosilica or vitriflable enamels of the lead bo'osllicate type, is used to define my improved enamels which contain lead oxide, silica and borlc oxide together with melted-in titanium dioxide and lithium oxide. These enamels may or may not contain alkali metal oxides in addition to lithium oxide.

The above description and examples of my invention are intended to be illustrative and not restrictive. Various modifications thereof and ,vaLriatlons therefrom which conform to the spirit- 'borosilicates of lead, said enamel containing as melted-in ingredients thereof, titanium dioxide and lithium oxide. Y

- 3. A sulfide-resistant, low-melting, lead silicate vitrifiable enamel suitable for decorating glassware containing as ingredients thereof, melted in titanium dioxide and lithium oxide.

4. A sulfide-resistant, low-melting, lead borosilicate vitrifiable enamel suitable for decoratin glassware containing as ingredients thereof,

.melted-in titanium dioxide and lithium oxide.

5. A. sulfide-resistant, low-melting vitriilable enamel suitable for decorating glassware comprising lead oxide, silica, melted-in titanium di- 16% b weight of titanium dioxide and 14%- by weight of lithium oxide, said oxides of titanium and lithium being melted into said vitriflable enamel composition.

8. A -sulfide-resistant, low-melting, vitrifiable enamel suitable for decorating glassware comprising 30-60% of lead oxide, 30-50% of silica, 1-6% of titanium dioxide and 1-4% of lithium oxide, said amounts being by weight based upon the total weight of the enamel, and said oxides ing 30-60% of lead oxide, 30-50% of Silica, boric oxide in an amount not exceeding 10%, 16% of titanium dioxide and 14% of lithium oxide, said amounts being by weight based'upon the total weight of the enamel, and said oxides being melted into said vitriflable enamel composition.

10. A sulfide-resistant, low-melting, vitrifiable enamel suitable for decorating glassware selected from the group consisting of silicates of lead and borosilicates of lead, said enamel containing 1-6% by weight of titanium dioxide, 14% by weight of lithium oxide, said oxides being melted into said vitrifiable enamel composition, and a ceramic pigment.

11'. A sulfide-resistant, low-melting, vitrifiable enamel suitable for decorating glassware obtainable by melting together lead oxide, silica, titanium dioxide and lithium oxide.

12. A sulfide-resistant, low-melting, vitrifiable enamel suitable for decorating glassware obtainable by melting together lead oxide, silica, boric oxide, titanium dioxide and lithium oxide.

13. A process for preparing sulfide-resistant, low-melting vitrifiable enamels suitable for decorating glassware comprising melting a composition comprising ingredients yielding in the molten state lead oxide, silica, titanium oxide and lithium oxide.

14. A process for preparing sulfide-resistant, 

